Touch panels have recently become widely adopted as the input device for high-end portable electronic products such as smart-phones and tablet devices. Although, a number of different technologies can be used to create these touch panels, capacitive systems have proven to be the most popular due to their accuracy, durability and ability to detect touch input events with little or no activation force.
The most basic method of capacitive sensing for touch panels is demonstrated in surface capacitive systems, for example as disclosed in U.S. Pat. No. 4,293,734 (Pepper, Oct. 6, 1981). A typical implementation of a surface capacitance type touch panel is illustrated in FIG. 1 and comprises a transparent substrate 10, the surface of which is coated with a conductive material that forms a sensing electrode 11. One or more voltage sources 12 are connected to the sensing electrode, for example at each corner, and are used to generate an electrostatic field above the substrate. When a conductive object, such as a human finger 13, comes into close proximity to the sensing electrode, a capacitor 14 is dynamically formed between the sensing electrode 11 and the finger 13 and this field is disturbed. The capacitor 14 causes a change in the amount of current drawn from the voltage sources 12 wherein the magnitude of current change is related to the distance between the finger location and the point at which the voltage source is connected to the sensing electrode. Current sensors 15 are provided to measure the current drawn from each voltage source 12 and the location of the touch input event is calculated by comparing the magnitude of the current measured at each source. Although simple in construction and operation, surface capacitive type touch panels are unable to detect multiple simultaneous touch input events as occurs when, for example, two or more fingers are in contact with the touch panel.
Another well-known method of capacitive sensing applied to touch panels can be found in projected capacitive systems. In this method, as shown in FIG. 2, a drive electrode 20 and sense electrode 21 are formed on a transparent substrate (not shown). The drive electrode 20 is fed with a changing voltage or excitation signal by a voltage source 22. A signal is then induced on the adjacent sense electrode 21 by means of capacitive coupling via the mutual coupling capacitor 23 formed between the drive electrode 20 and sense electrode 21. A current measurement means 24 is connected to the sense electrode 21 and provides a measurement of the size of the mutual coupling capacitor 23. When a conductive object such as a finger 13 is brought within close proximity of both electrodes, it forms a first dynamic capacitor to the drive electrode 27 and a second dynamic capacitor to the sense electrode 28. The effect of these dynamically formed capacitances is manifested as a reduction of the amount of capacitive coupling in between the drive and sense electrodes and hence a reduction in the magnitude of the signal measured by the current measurement means 24 attached to the sense electrode 21. As is well-known, by arranging a plurality of drive and sense electrodes in an array, such as a two-dimensional matrix array, this projected capacitance sensing method may be used to form a touch panel device. An advantage of the projected capacitance sensing method over the surface capacitance method is that multiple simultaneous touch input events may be detected. However, in spite of the multi-touch capabilities of the projected capacitive method, it has some significant limitations. For example, it cannot be used to detect the force of touch input and is unable to detect touch input from non-conductive objects such as a plastic stylus or pen.
In order to overcome these limitations, hybrid systems incorporating force sensing devices into projected capacitive touch panels have been proposed. For example, “Metal-polymer composite with nanostructured filler particles and amplified physical properties”, Applied Physics Letters 88, 102013 (2006), discloses a force sensitive material which may be used to form a ring around the periphery of the touch panel. Alternatively, U.S. Pat. No. 6,492,979 (Kent, Dec. 10, 2002) describes a touch panel system incorporating discrete force sensing devices. A force sensor may also be formed in the touch sensor electrode layer: for example, U.S. Pat. No. 5,915,285 (Sommer, Jun. 22, 1999) describes strain gauges formed from Indium Tin Oxide, and inter-digitated amongst the touch sensor electrodes. However, these systems are limited in that they cannot individually measure multiple forces applied at different points.
A method of simultaneously measuring multiple separate touches, together with their associated forces, is proposed in U.S. Pat. No. 7,538,760 (Hotelling, May 26, 2009). This patent describes compressible structures of capacitive sensor electrodes, such as that shown in FIG. 3. The structure of FIG. 3 employs a layer of projected capacitive sense electrodes 410 and a first set of drive electrodes 420 to determine the location of each touch, in the manner of a conventional projected capacitive touch sensor. These sense electrodes 410 and drive electrodes 420 will typically run in orthogonal directions, and may be formed on opposite sides of a sensor substrate 430. A spring structure 440 separates the sense electrodes 410 from a second set of drive electrodes 450. A protective cosmetic layer 460 lies on top of the first set of drive electrodes, and the entire sensor structure is supported by a support substrate 470. The force applied to a point on the sensor influences the local compression of the spring structure, and therefore changes the local capacitance measured between the sense electrodes 410 and the second set of drive electrodes 450. This local capacitance is indicative of the local touch force.
A similar structure, shown in FIG. 4, is disclosed in U.S. Pat. No. 7,511,702 (Hotelling, Mar. 31, 2009). This structure employs a protective cosmetic layer 460 on top of a first transparent layer 480 and a second transparent layer 482, separated by a spring structure 440. The first transparent layer 480 has a set of drive electrodes 486, whilst the second transparent layer 482 has a set of touch sense electrodes 488 and a set of force sensing electrodes 490. Touch forces compress the deformable layer, increasing parallel plate capacitances that are measured using the force sensing electrodes 490. Simultaneously, the touch sensing electrodes are used to detect the presence of objects above the touch sensor.
In any touch sensor that relies upon parallel plate capacitances between drive electrodes and force sensing electrodes, the parallel plate capacitances may limit the frequency at which the sensor can operate. This is because, in conjunction with the resistance of the electrodes themselves, the parallel plate capacitances form electrical low pass filters. If the frequency at which the proximity sensor can operate is limited, then fewer measurements are obtained in a given amount of time. This translates either to a reduction in the signal to noise ratio, in which case smaller or more distant objects may not be detected, or to a reduction in frame rate, in which case the system will not accurately perceive rapidly changing input stimuli.